I wrote an article about a paper I read in the journal Science a few weeks ago – the article was about Rotary Photon Drag Enhanced by A Slow Light Medium. I got two handfuls of emails about the article, so I got in contact with one of the original paper’s editors, Sonja Franke-Arnold. When you have questions, it’s best to go to the source!

JimOnLight.com: Hi Sonja, welcome to JimOnLight.com! I’m very interested in your research, and we’ve gotten a lot of interesting response to the post I wrote on your paper, “Rotary Photon Drag Enhanced by a Slow-Light Medium.” Can you take a moment and give us a bare-bones layperson’s look at what you and your team has discovered? What exactly has happened here in your experiment?

Sonja Franke-Arnold: We were wondering how the world looks like through a spinning window! About 200 years ago Augustin-Jean Fresnel predicted that light can be dragged if it travels through a moving medium. If you were to spin a window faster and faster, the image would actually be slightly rotated as the light is dragged along with the window. However, this effect is normally only some millionth of a degree and imperceptible to the eye.

We managed to increase the image rotation by a factor of about a million to an easily noticeable rotation of up to 5 degrees. This happened by slowing the light down to roughly the speed of sound during its passage through the “window” (in fact a ruby crystal). The light therefore spent a longer time in the ruby rod and could be dragged far enough to result in an observable image rotation.

JimOnLight.com: Can you explain the significance of the wavelength of light you used? Why was 532nm (green) used for the experiment?

Sonja Franke-Arnold: This wavelength excites a transition within the ruby crystal (the same that is also used in ruby lasers). Light at 532nm is absorbed and excites an atomic level with a very long (20 millisecond) lifetime. This allows to “store” the energy of the photon as an internal excitation of the rotating ruby crystal – generating slow light.

JimOnLight.com: Tell me about the significance of the shape of the coherent beam in the experiment – was the shaped beam simply to observe a change in the image, or was a different purpose considered?

Sonja Franke-Arnold: We used an elliptical light beam for two reasons, one of these is to define the image rotation angle as you suggested. The elliptical beam travelling through the spinning ruby rod however also plays an important part in making the slow light itself: At any particular position of the ruby, the elliptical light – spinning with respect to the ruby – looks like an intensity modulation. The varying intensity produces a large refractive index of about one million which slows the light down from the speed of light to roughly the speed of sound – a method pioneered by our co-worker Robert Boyd.

JimOnLight.com: Could you give a few examples of uses for this discovery? How can the general populous relate to what this discovery really means for light and photonics?

Sonja Franke-Arnold: For me, the main highlight was that we managed to observe a 200 year old puzzle – that images are indeed dragged along with rotating windows. We are now working on possible applications in quantum information processing: our image rotation preserves not only the intensity but also the phase of the light and could therefore be used to store and rotate quantum images. Access to the angle of an image could allow a new form of image coding protocol.

Thanks so much, Sonja! Very cool paper for those of us nerds out here!

Remember that scene in the Jody Foster movie called Contact when they got all of those drawings of “the machine?” There was a part of the movie where Ellie realized that the images were encoded somehow, and the key to encoding them was by looking at them in three dimensions. Remember that minute little detail?

I read an article on this just the other day, and after I read the entire article in the journal Science, I really want to share the gist of this thing with you all. It totally reminds me of this for some reason. I was explaining this all to a friend on Skype, and I got tired of typing, and then the researcher slice of my brain started going ape-sh**. Pardon me.

First, read the abstract of the article written by Sonja Franke-Arnold (School of Physics and Astronomy (SUPA), University of Glasgow, Scotland), Graham Gibson andRobert W. Boyd (Department of Physics, University of Ottawa, Ottawa, Canada), and Miles J. Padgett (The Institute of Optics and Department of Physics and Astronomy, University of Rochester, Rochester, NY):

Transmission through a spinning window slightly rotates the polarization of the light, typically by a microradian. It has been predicted that the same mechanism should also rotate an image. Because this rotary photon drag has a contribution that is inversely proportional to the group velocity, the image rotation is expected to increase in a slow-light medium. Using a ruby window under conditions for coherent population oscillations, we induced an effective group index of about 1 million. The resulting rotation angle was large enough to be observed by the eye. This result shows that rotary photon drag applies to images as well as polarization. The possibility of switching between different rotation states may offer new opportunities for controlled image coding.

Ok, got it? Yeah, read it a few times, but generally the concept of the experiment is pretty simple, and the results are very interesting! What these folks were doing was shining a shaped, collimated beam of light through a spinning ruby disk rotating at a given speed – in this case a maximum of 30 cycles per second. The ruby disk causes a bit of “drag” on the photons travelling through it, causing the light to refract and exhibit some interesting behavior. Check out this little video, from the paper and from the journal Science:

Ruby has a heavy Index of Refraction, which means the light is slowed down (refracted) at a rate of X when it leaves the air and enters the ruby itself. If you imagine the 1.0 value of the Index of Refraction as how light travels through regular ol’ air (and not taking into account humidity, pollution, or any of that schtuff), anything greater than 1.0 is refracting. Diamond has an Index of Refraction of about 2.42, and Ruby’s Index of Refraction is about 1.77. Ruby refracts less than diamond. Make sense if you didn’t already get it?

Here’s the weird thing – Ruby is not what we consider isotropic – meaning that no matter what the incidence angle is and no matter what the orientation of the crystal is, the light travels through the crystalline matrix equally as it travels through the medium. Glass, sodium chloride crystals, and a lot of polymers exhibit this kind of “perfect” structure. Sodium chloride is basically a cubic structure, relatively perfectly bonded in a cube matrix. Ruby, on the other hand, is an anisotropic crystalline structure, meaning that there are more than one axes that are different within the structure of the crystal matrix.

Here’s a good image of the difference between an isotropic and anisotropic crystal structure, optically, from Olympus America’s Microscopy Resource Center. Figure A is a sodium chloride crystal, which is isotropic. Figure B is a calcite crystal, which has calcium ions and carbonate ions in it. Calcite is anisotropic. Check it:

Ok – now if you think of a crystal structure with light shining through its matrix, and the light is going to pass through two different planes of refraction, essentially – what do you expect to happen to one beam of light as it enters the anisotropic crystal structure and slows down?

Who said it’s going to split into two beams? (DJ Lemma, pout your hand down, I know you already know the answer!) You’d be correct – the incident beam splits into two beams, each sort-of along that individual crystal plane. Take a look at this image of a calcium carbonate crystal, and how it is creating a double image:

This phenomenon is called birefringence. Deep breath – bi-re-frin-gence. Ruby, the gem used in the experiment, is also an anisotropic crystal, and it exhibits traits of birefringence.

So, imagine taking that birefringent crystal disk, spinning it at a relatively high rate (30 Hz), and shining a very specific wavelength of light (ie, a laser), that is in a certain shape through the ruby disk as it spins. A bunch of stuff was discovered with this experiment, all related to the image. The generalities of the experiment, as I paraphrase, is that the group shone a very bright laser with a square-ish shape through the ruby disk, noted the position that the laser had ont he other side of the ruby disk after it was on the other side of the disk. When you shine a shaped laser beam at 532 nanometers (green) through a spinning ruby disk (which is a very slow-light medium, slowing the light down to just a few tens of meters per second) spinning at a rate between not spinning and 30 rotations per second, the image refracts from about a third of a degree to about ten degrees as the ruby disk increases from slow revolutions per second to thirty revs per second.

What a crazy experiment, huh? I needed a good dose of photonics and optics in my Thursday!

The ramifications of this experiment have to do with encoding images with extra data – if you can imagine an image that has more information in it depending on which way the image is spinning, that is some trippy Minority Report shizzyhizzle. “Oh, you’ve stolen my image! But since you don’t know which wavelength to use and at which speed to spin the image, you’ll never decode my super secret plans of world domination!!!”